1. Field of the Invention
The present invention relates generally to optical radiometry and, more particularly, to an apparatus for measuring a spectral radiant flux or a total flux of a lamp by the use of an integrating photometer, which is in wide practical use today in the field of optical radiometry.
2. Description of the Prior Art
In development or quality control of incandescent lamps or various discharge lamps, an accurate measurement of the total flux value of a lamp is especially important. In measuring the total flux of the lamp, an integrating photometer is generally used. The integrating photometer is a device which comprises an integrating sphere having a light sensing window defined in a wall thereof, a spectrometer for measuring light transmitted through the window, a lamp holder for holding and lightening a lamp to be measured inside the integrating sphere, and a light shielding plate for shielding that portion of light, emitted from the lamp when the lamp is turned on, which travels directly towards the window. The entire interior wall surface of the integrating sphere is a white diffusion surface so as to permit the interior wall surface to exhibit a uniform illuminance, when radiated by the lamp, by the effect of interreflection taking place inside the integrating sphere.
A total flux measuring apparatus currently in use measures the total flux of the lamp by comparing it with a known total flux value of a standard lamp. Because the lamp is turned on inside the integrating sphere during the measurement, the integrating photometer can be used in a bright room. For this reason, the integrating photometer is utilized not only in total flux measurement, but also in spectral distribution measurement wherein a spectrometer is mounted on the light sensing window. Also, the integrating photometer is widely used in lamp manufacturing factories or in lamp development sections.
The integrating photometer, however, has the following problems: In the first place, a white diffusion layer coated on the interior wall of the integrating sphere does not have a 100% reflectance and has such a wavelength selectivity that the reflectance varies according to the wavelength. Secondly, the presence of the light shielding plate inside the integrating sphere hampers sufficient interreflection. Yet, the wavelength selectivity or the extent to which the interreflection occurs varies according to the lamp to be measured.
For these reasons, if the lamp to be measured differs from the standard lamp in spectral distribution, distribution of luminous intensity, self-absorption, or the like, the integrating photometer is apt to give rise to a considerable measurement error and, hence, the integrating photometer is generally used as a practical instrument.
On the other hand, in order to compensate for the measurement error, attempts have hitherto been carried out to find out the characteristic of the integrating photometer.
Japanese Laid-open Patent Publication No. 3-233338 discloses a method of measuring the reflectance of an interior wall surface of the integrating sphere with the use of a light absorption sheet. This method is utilized to find out the spectral characteristic of the integrating photometer.
Also, a technique of analyzing interreflection characteristics inside the integrating photometer has hitherto been reported ("Analysis of an Integrating Sphere by Computer Simulation" written by Yoshihiro Ohno in 1987).
As described hereinabove, although analysis of the basic characteristic of the integrating photometer has been proposed up to this time, no attempts have been made to the application thereof to the total flux measurement.